The present invention relates to hydropneumatic actuator devices and particularly to brake actuators for railway vehicles operating in passenger transit service. These brake actuators may be either tread or disc brake units.
Typically, these brake units employ a light spring to bias the brake shoe into constant engagement with the wheel tread or disc during brake release, in order to generate, by friction, sufficient heat to prevent the accumulation of ice and snow on these brake parts during winter time and to assure a prompt response of the brakes. These so-called "zero-clearance" arrangements, in addition, offer the advantages of simple construction and accordingly low-cost manufacture.
Recent tests, however, have shown that the drag of the brake shoes during periods of brake release requires a considerable expenditure of train power, particularly where the train consists of more than one or two cars. With today's high cost of fuel, the consumption of power needed to overcome brake drag becomes increasingly critical and can offset any advantages heretofore realized.
Accordingly, "positive clearance" maintaining arrangements have been developed which employ friction elements to achieve the "positive clearance" operating characteristic. Typically, these friction elements consist of an aluminum split ring, which is designed to exert a radial force against the cylinder bore in which a power piston is operative to apply braking force. This principle of operation has not been completely successful, however, due to the fact that rapid wear of the friction element itself occurs with resultant cylinder bore damage and accordingly a short service life.